1. Technical Field
The present disclosure relates to a field equipment of two-wire and, more particularly, to a field equipment of two-wire capable of achieving a size reduction, a low cost, and low power consumption.
2. Background Art
The field equipment of two-wire constitutes a part of the plant facilities for use in process control, and measures a flow rate, a pressure, a temperature as a process quantity (measured object) and outputs this process quantity to a controller. Then, the controller executes the process control such as control of the flow rate, the pressure, the temperature, or the like. The field equipment of two-wire is supplied with an electric power from an external power source. Then, an electromagnetic flowmeter 18 of two-wire system as one of the field equipment will be described with reference to FIG. 6 hereunder.
The electromagnetic flowmeter 18 is constructed by a detecting section 4 for detecting a signal related to process quantity, a signal processing section 7, insulating sections 8 to 12, a calculation control section 13, a DC-DC converter 14, an exciting section 15, a current output section 16, etc.
The electromagnetic flowmeter 18 applies a magnetic field to a measured fluid that flows through a pipe of the detecting section 4, then detects an electric signal generated in the measured fluid based on the magnetic field, and then calculates a flow rate of the measured fluid based on the electric signal to output the flow rate.
One output terminal T1 of a pair of output terminals of the electromagnetic flowmeter 18 is coupled to a positive terminal (+) of an external DC power supply 20, and the other output terminal T2 is coupled to a negative terminal (−) of the external DC power supply 20 via a resistor 19.
The electromagnetic flowmeter 18 takes in a current corresponding to a flow rate value to be calculated (e.g., in a range from 4 mA to 20 mA) from the positive terminal (+) of the DC power supply 20 to said one output terminal T1, and outputs the current from the other output terminal T2 to the negative terminal (−) of the DC power supply 20 via the resistor 19. Accordingly, the electromagnetic flowmeter 18 is supplied with the electric power given by the DC voltage and the current of the DC power supply 20.
A controller 21 is coupled across the resistor 19 and measures the current being output from the electromagnetic flowmeter 18 as the voltage across the resistor 19, and then converts this measured voltage to a flow rate value, and thus executes flow rate control.
Said one output terminal T1 coupled to the positive terminal (+) of the DC power supply 20 is coupled to a first power supply line L1. The power supply terminals of the calculation control section 13, the input side (SW control circuit) of the DC-DC converter 14, the exciting section 15, the current output section 16, and the insulating sections 8 to 12 are coupled to the first power supply line L1.
A connection point of the current output section 16 and the output current detection resistor 17 is coupled to a first common potential L2. The reference potential terminals of the calculation control section 13, the input side (SW control circuit) of the DC-DC converter 14, the exciting section 15, the current output section 16, and the insulating sections 8 to 12 are coupled to the first common potential L2. Then, the calculation control section 13, the DC-DC converter 14, the exciting section 15, the current output section 16, and the insulating sections 8 to 12 are supplied with an electric power from the first power supply line L1.
A power supply terminal on the output side of the DC-DC converter 14 is coupled to a second power supply line L3. The power supply terminals of an amplifier 5, an AD converter 6, and the insulating sections 8 to 12 are coupled to the second power supply line L3.
A reference potential terminal on the output side of the DC-DC converter 14 is coupled to a second common potential L4. The reference potential terminals of the amplifier 5, the AD converter 6, and the insulating sections 8 to 12 are coupled to the second common potential L4. Then, the amplifier 5, the AD converter 6, and the insulating sections 8 to 12 are supplied with an electric power from the output (the second power supply line L3) of the DC-DC converter 14.
The detecting section 4 is constructed by an exciting coil 1, electrodes 2 and 3, a pipe (not shown) that flows through the measured fluid, and the like. Also, the exciting section 15 is coupled to the calculation control section 13 and the exciting coil 1, and feeds an excitation current to the exciting coil 1 based on a control signal L8 from the calculation control section 13.
The exciting coil 1 generates a magnetic field in the pipe of the detecting section 4 and applies the magnetic field to the measured fluid in the pipe. Thus, an electric signal (an induced voltage) that is proportional to a magnetic flux density of the magnetic field and a flow rate of the measured fluid is generated in the measured fluid flowing through the pipe. Then, this electric signal is detected by the electrodes 2 and 3 arranged in the pipe.
The signal processing section 7 is constructed by the amplifier 5 and the AD converter 6. The amplifier 5 is constructed by a differential amplifier, a noise removal filter, an empty sensing section (not shown) for sensing whether or not the measured fluid is present in the pipe.
The differential amplifier of the amplifier 5 receives signals L6, L7 related to process quantity and detected by the electrodes 2, 3, and outputs a signal being obtained by amplifying differentially the signals to the AD converter 6. This differentially amplified signal is in proportion to a flow rate of the measured fluid.
The AD converter 6 receives the control signal output from the calculation control section 13 via the insulating section 8, and performs AD-conversion (Analog-Digital signal conversion) on the differentially amplified signal based on this control signal. The AD converter 6 outputs the AD-converted signal to the calculation control section 13 via the insulating section 9 after the AD conversion is completed.
Also, when the output of the differential amplifier of the amplifier 5 is saturated, the amplifier 5 receives the control signal output from the calculation control section 13 via the insulating section 10, and changes (reduces) an amplification factor of the differential amplifier based on this control signal.
Also, the amplifier 5 receives the control signal output from the calculation control section 13 via the insulating section 11, and causes the noise removal filter consisting of a resistor and a capacitor to discharge a charge accumulated in the capacitor based on the control signal. Also, the amplifier 5 receives the control signal output from the calculation control section 13 via the insulating section 12, and causes the empty sensing section to perform an empty sensing function based on the control signal.
The insulating sections 8 to 12 have an interface function of electrically insulating the circuits whose reference potentials are different (the first reference potential L2 and the second reference potential L4) mutually and converting the signals such that these circuits can transmit/receive the signal mutually.
Here, the electrodes 2 and 3 might be grounded via the measured fluid having an electric conductivity in the pipe and the piping (not shown) coupled to the detecting section 4. Also, the negative terminal (−) of the DC power supply 20 might be grounded.
Unless the insulating sections 8 to 12 are provided, a loop current flows through the electromagnetic flowmeter 18, the measured fluid, the piping, and the DC power supply 20 when the electrodes 2 and 3 and the negative terminal (−) of the DC power supply 20 are grounded. Then, a common mode voltage is generated by this loop current, and an error arises in the output of the electromagnetic flowmeter 18. The insulating sections 8 to 12 are provided to prevent this loop current.
The DC-DC converter 14 might be an insulation-type DC voltage converting circuit of the inverter system. The DC-DC converter 14 converts the DC voltage on the first power supply line L1 into the AC voltage by the SW control circuit, then upconverts or downconverts the AC voltage by a transformer, and then rectifies the resultant AC voltage by a diode and a capacitor. Thus, the DC voltage on the first power supply line L1 is converted into the DC voltage on the second power supply line L3.
A circuit group coupled to the first power supply line L1 and the first common potential L2 and a circuit group coupled to the second power supply line L3 and the second common potential L4 are electrically insulated mutually by the DC-DC converter 14.
The calculation control section 13 calculates a flow rate value of the measured fluid by multiplying the AD-converted signal by an inner diameter of the pipe of the detecting section 4.
The calculation control section 13 outputs a PWM signal (pulse-width modulated signal) L9 with a duty factor, which is in proportion to a calculated value of the flow rate, to the current output section 16.
An output current detection resistor 17 detects an output current to the resistor 19 as a voltage L5. The current output section 16 outputs a current, which is in proportion to a calculated value of the flow rate, by comparing the voltage that is obtained by smoothing the PWM signal L9 with the voltage L5 (see e.g., JP-A-2002-340638).
Often a large number of the field equipments of two-wire such as the electromagnetic flowmeter 18 of two-wire system are installed in the field site where the pipe through which the measured fluid flows are provided. Therefore, it is preferable that such a field equipment of two-wire should be small in size including an installing space and low in cost. Also, it is demanded in some cases that the field equipment of two-wire has a low power consumption (e.g., a current consumption of the equipment is 3.8 mA or less) to satisfy the intrinsically safe explosion-proof standards such as IEC60079-11.
However, as described in FIG. 6, the field equipment of two-wire needs many insulating sections. This insulating section is constructed by a transformer, a photocoupler that needs an emitting signal driving transistor and a receiving light signal detecting transistor, or the like. In this manner, because the number of circuit components is increased in the insulating section, it is difficult to manufacture the field equipment of two-wire in a small size, at a low cost, and at low power consumption.